Karst Geology and Hydrology

Sink holes, swallows, and caves are karst features. Karst is the term used for the many expressions left on or in bedrock from of water action. Such features can have substantial impact on farming, construction, groundwater, and surface waters. Karst allows unfiltered water, soils, and surface debris entry into the subsurface. The solids settle in the cavities and caverns, while water and soluble materials flow deep underground to the Great Lakes.

Pleistocene glaciers scraped across the bedrock of Michigan, removing most surficial (near the surface) karst features. The last glacier began its retreat from Southern Michigan 12,000 years ago; and, 9,000 years ago, it finally left Northern Michigan. Most karst exposed features have developed since then.

The karst development is in a group of rocks called the Detroit River Group (named for an exposure near Detroit). It is over 275 meters (900 feet) below the surface at Alpena, and nearly 30.5 m (100 ft) below surface at Rogers City. It is composed of a sequence of limestones, shales, and evaporate minerals (gypsum and salt). Surface and ground waters invaded the formation through open faults. As the evaporites were removed by solution, the formations above collapsed and brecciated (broke up). Sinkholes developed from uneven collapse and additional solution at swallow areas. Many sinks are visible along a fault line as the Shoepac Lake-Rainy Lake and Sunken Lake-to-Misery Bay line of sinkholes, and the sinkhole lakes of Presque Isle and Alpena counties. Northern Michigan sinkholes also exist in the northeast corner of Otsego County and west of Wolverine, with a few features on the west side in the Charlevoix-Grand Traverse-Leelanau-Manistee area.

Outflow of the system is toward the nearest of either lakes Huron or Michigan. Currently, discharge is known only at El Cajon Bay, Middle Island, and a remote sinkhole 24 kilometers (15 miles) from Alpena into Lake Huron. With a karst path that zigzags along caverns and fractures deep underground, contamination would be difficult to nearly impossible to trace or capture.

The system began forming millions of years ago, but only sinkholes "active" since the retreat of the glacier about 10,000 years ago have become visible at the surface. The last glacial advances plugged or covered over the older features. However, the older features may be actively swallowing groundwater as it filters through the cover and collapsed formations.

To better understand the natural impact to lakes Michigan and Huron, and potential human impacts, we must study the geological setting, origin, cause, hydrology, and remediation of sinkholes. It is important to understand the recent geological and hydrological history to more completely understand the current karst geology and hydrology.

Tyrone Black has worked for the Geological Survey for over 31 years in oil and gas regulation. During his free time he has studied, observed, and written several papers on Michigan's karst.